1. Field of the Invention
The present invention relates to thyristor-type switches that can be set conductive when a voltage with a suitable polarity is present at their main terminals and when a pulse is applied to their gate, and that remain conductive as long as a voltage with a suitable polarity is applied to their main terminals. The present invention relates more particularly to amplifying-gate thyristors with high hold current which can be advantageously controlled by a low triggering current.
2. Discussion of the Related Art
FIG. 1A schematically represents a circuit for an amplifying-gate thyristor including a main thyristor T1 and amplifying thyristor T2. An anode A and a cathode K of the amplifying-gate thyristor correspond to the anode and to the cathode of the main thyristor T1. An amplifying thyristor T2 has its anode connected to anode A and its cathode connected to the gate of the main thyristor T1. The gate of thyristor T2 constitutes the control terminal G of the amplifying-gate thyristor T.
FIG. 1B shows the waveform of the current-voltage characteristic curve of an amplifying-gate thyristor (which also corresponds to a conventional thyristor).
When a voltage V1 is applied between the anode and cathode of a thyristor and a pulse is applied to gate G, the thyristor becomes conductive. First, the current I increases at a substantially constant voltage and, then, the voltage rapidly drops. The operating point F moves along a practically vertical curve C with a low voltage drop across the thyristor. For example, if the pulse applied to the gate is applied at the beginning of a positive half-period of a periodic signal, the current first increases, then decreases. When the operating point F reaches a minimum value corresponding to a hold current I.sub.H, the thyristor reverts to its off state. A gate voltage must be applied again to the thyristor T to reset it to its conductive state. Thus, the thyristor T is conductive as long as the circuit is adapted to let a current higher than the hold current flow through the thyristor.
In some amplifying-gate thyristor applications, it is desired to have a high value for the hold current IH. The thyristor should cut off when the current decreases below a relatively high value.
In the state of the art, SGS-Thomson Microelectronics markets, under reference TN22, thyristors that are controlled by a 1.5 mA-gate current, have an avalanche voltage within a range of 1000-1600 volts and a hold current higher than 175 mA.
Among the features required in order to obtain a high hold current, the thyristor cathode must have a relatively high emitter shorting hole density. Therefore, it is possible, using conventional techniques, to design a thyristor T1 adapted to have a relatively high hold current. However, it is impossible to indefinitely increase the shorting hole density and accordingly the value of the hold current. Indeed, an increase in the shorting hole density corresponds in particular to an increase in the triggering current of the thyristor and its voltage drop in the conductive state. Therefore, increasing the hold current decreases the capability to be triggered and to withstand high current flows.
Since the amplifying thyristor T2 must be sensitive enough to permit triggering at a low control current of the amplifying-gate thyristor, it unavoidably has a relatively low hold current. Thus, to manufacture an amplifying-gate thyristor with a relatively high hold current, the circuit designer uses various conventional techniques so that, once the thyristor T2 triggers the thyristor T1, only thyristor T1 remains conductive. Once thyristor T1 is triggered, it is sufficiently conductive with respect to thyristor T2 that the current in thyristor T2 becomes lower than its low hold current and it turns off.
Thus, as illustrated in FIG. 1A, once the current is established in thyristor T, the whole anode current I will flow through the main thyristor T1, and the current in the amplifying thyristor T2 will be zero. Accordingly, at the turn off, only the high hold current of thyristor T1 intervenes since thyristor T2 is already turned off.
As indicated above, for a thyristor with a given size, the possibility of increasing the value of the hold current is limited in order not to impair the other features of the thyristor.